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Cheap Thoughts: variable-current EV charging

I’ve been mostly happy with our Siemens Level 2 EV charger. It’s simple to use with only two buttons, which I rarely need to press. Still, there is one feature the Siemens does not offer that I wish it had: the ability to adjust the current used based on my electricity rate plan’s Time of Use schedule.

Duke Energy offers a Time of Use – Demand (TOU-D) electric plan (which I’ve discussed in-depth before), meaning an electric customer gets socked with high fees based on how much electricity gets used at the same time.
Here’s where a more clever charging station becomes useful. Let’s say my usual monthly peak demand is 8 kW. I pay roughly $4 per kW demand so I have a powerful incentive to stay within my electric “budget.” My AC uses 3 kW and my stove uses 1 kW, leaving me 4 kW of my 8 kW monthly budget to play with. Now, with my Siemens charger I get all or nothing: it’s either pumping 7 kW into my car or it’s offering a trickle of juice. When I’m paying off-peak rates at night, I’m fine with 7 kW being sent to my car. But what if I need a quick charge during on-peak times? I have two choices: either I give my car a full-blown 7 kW charge (Level 2, 220 volts) or a 1.5 kW trickle charge (Level 1, 110 volts), which takes forever. If I add 7kW to my 3 kW AC and 1 kW stove, I’m up to 11 kW and I blow out my electric bill.

If I could get the charger to automatically offer only up to 4 kW during on-peak times and save the full power until the cheaper rates kick in, I get the best of both worlds: I can get a quicker charge than the lowly Level 1 and still stay within my demand budget. This could shave $20 or more per month from our electricity bill. More importantly, I would no longer have to worry about what electricity rates are in effect at the time I plug in my car – the charger would adjust its charging rate for me.

It would be good to see more commercially-available electric charging stations have more flexibility in dealing with changing electricity rates as the one-size-fits-all approach doesn’t match the market. Once again, open source leads the way!